when I book online through Rail Europe for the TGV, our seats are automatically assigned to us when I get my tickets through the post. In my experience, no-one pays the slightest bit of attention to assigned seats on the TGV, and for that 9-minute journey it doesn't matter anyway .
I've only sat in upper deck (automatically assigned to me) but there's not much to see - or there may be but I'm normally only thinking disney by then!!!
You get yourself settled at airport but then before you're at Disney you're up and moving to exit so maybe only 5/6 mins in the seat once train gets going.